As electromagnetic wave noise generated from printed wiring boards, electronic parts and the like may affect other electric circuitry and electronic parts, and may cause malfunctions and the like, it is necessary to shield out electromagnetic wave noise. Consequently, printed wiring boards are being provided with an electromagnetic wave shielding function.
In conjunction with the miniaturization and multifunctionalization of electronic equipment equipped with printed wiring boards, the allowable space in printed wiring boards has been narrowing. Consequently, there is demand for reduced thickness in printed wiring boards, and also for reduced bending radius in flexible printed wiring boards. There is also demand for achievement of wiring functions without loss of reliability of the electromagnetic wave shielding function even in a variety of usage environments.
As a printed wiring board with an electromagnetic wave shielding function, for example, the following has been proposed.
(1) A flexible printed wiring board wherein an undercoat layer, a shielding layer coated with a conductive paste containing a metal powder, and an overcoat layer are sequentially provided on a copper foil wiring circuit with a heat-resistant plastic film surface, and wherein the shielding layer and the ground pattern of the copper foil wiring circuit are electrically connected through the undercoat layer at appropriate interval (Patent Document 1).
(2) A flexible printed wiring board wherein an electromagnetic wave shielding film—in which a metal thin-film layer and a conductive adhesive layer containing a metal filler are sequentially provided on one surface of a coverlay film—is mounted on a base film, on which an insulating layer that imparts insulation excluding a portion of a ground circuit among the printed circuits is provided, so that the conductive adhesive layer adheres to the insulating layer and a portion of the ground circuit (Patent Document 2).
However, the flexible printed wiring board of (1) has the following problems.
(i) The shielding layer containing the metal powder is fragile due to numerous interfaces between heterogeneous materials, and lacks sufficient strength against repeated bending of the flexible wiring board.
(ii) The undercoat layer is required in order to maintain insulation between the shielding layer and the copper foil wiring circuit excluding a portion of the ground pattern, thickening the flexible printed wiring board.
(iii) It is necessary to form a through hole in part of the undercoat layer in order to electrically connect the shielding layer and a portion of the ground pattern, and the processing of the through hole requires time and labor.
(iv) As the shielding layer is exposed at the circumferential end faces of the flexible printed wiring board, corrosion occurs due to infiltration of moisture and contact with oxygen, increasing the resistance values, and attenuating the electromagnetic wave shielding function. Therefore, the reliability of the electromagnetic wave shielding function is poor.
(v) As the shielding layer is composed of curing resin obtained by curing the conductive paste, the shielding layer cannot be etched into a desired shape using etching liquid.
In addition, the flexible printed wiring board of (2) has the following problems.
(i) The conductive adhesive layer containing the metal filler is fragile due to numerous interfaces between heterogeneous materials, and lacks sufficient strength against repeated bending of the flexible wiring board.
(ii) An undercoat layer is required in order to maintain insulation between the conductive adhesive layer and the printed circuits excluding a portion of the ground circuit, thickening the flexible printed wiring board.
(iii) It is necessary to form a through hole in part of the insulating layer in order to electrically connect the conductive adhesive layer and a portion of the ground circuit, and take a lot of trouble with the working of the through hole.
(iv) As the metal thin-film layer is exposed at the circumferential end faces of the flexible printed wiring board, corrosion occurs due to infiltration of moisture and contact with oxygen, increasing the resistance values, and attenuating the electromagnetic wave shielding function. Therefore, the reliability of the electromagnetic wave shielding function is poor.
(v) As the material of the metal (silver) thin-film layer and the material of the printed circuits (copper) are different, etching cannot be conducted with the same etching liquid. Consequently, the etching equipment and etching liquid must be respectively prepared for etching of the metal thin-film layer and etching of the printed circuits.